Fix withAllowedTimestampSkew deprecation warning

[nevillelyh]

Need to look at the implications.

def timestampBy(f: T => Instant, allowedTimestampSkew: Duration = Duration.ZERO): SCollection[T] =
    this.applyTransform(WithTimestamps.of(Functions.serializableFn(f))
      .withAllowedTimestampSkew(allowedTimestampSkew))

https://issues.apache.org/jira/browse/BEAM-644

[LukeDefeo]

Im trying to understand what allowed timestamp skew is. Is it the maxium allowed duration between event time and processing time?

[Fokko]

This medium post explains it quite clearly: https://medium.com/@erik.b.ferm/combining-bounded-pcollections-with-custom-timestamps-in-apache-beam-b0fd911b2ac